1. Field of the Invention
The present invention relates to monitoring the communication quality of a communication path set up by an ATM (Asynchronous Transfer Mode) switch that provides, in addition to virtual connection (VC) service, permanent VC (PVC) service and point-to-multipoint connections.
2. Description of the Related Art
In the VC service, an intraswitch communication path to a destination terminal and, when necessary, a communication path to other switches, is set up in response to a request from an originating terminal. In the PVC service, an intraswitch communication path and an interswitch communication path are set up in accordance with an input to a maintenance console connected to the switch.
In a conventional ATM switch, when setting up a VC communication path, transmission of user cells through the communication path is permitted when the connection result of a communication control signal and the result of the processing of the communication path setup within the switch are normal. Whether the communication path is functioning normally is checked by inserting an OAM (Operations, Administration, and Maintenance) cell during the transmission of user cells after the communication is started. On the other hand, when setting up a PVC communication path, transmission of user cells through the communication path is permitted when the result of the processing of the communication path setup within the switch is normal. Whether the communication path is functioning properly is checked by inserting an OAM cell during the transmission of user cells after the communication is started.
Accordingly, since cell flow verification is not performed when setting up a VC communication path, there may arise situations where cells are not actually flowing within the ATM switch even though the communication path has been established and permitted for use by the user, or where cells are not flowing through a line between ATM switches. Such situations cannot be identified until a fault condition is detected by on-demand transmission of an OAM cell after the communication path has been established. The need therefore arises for a method that can verify cell flow when setting up a VC communication path. For the PVC service also, as in the VC service, a method becomes necessary that can verify cell flow when setting up a communication path.
Further, once the communication path has been set up, cell flow quality of the communication path is monitored by monitoring the condition of user cell flow by using a performance monitoring (PM) function, etc. However, in the conventional system, since OAM cells are sent out only in a sporadic manner, it has not been possible to determine whether the quality of service (QOS) defined as statistic parameters such as cell loss ratio is satisfied or not. Furthermore, if a deterioration of the cell flow quality of the communication path has been detected by a sporadic OAM cell transmission test, the function has been only to notify system administrator, and the result of the detection has not been reflected in the selection of a new route, giving rise to the possibility of routing a new path along the same line that includes the deteriorated path, and resulting in quality degradation also on the new path. Moreover, there have been cases where, when a PVC is attempted to be set up along that route, the PVC is set up as attempted, leading thereafter to quality degradation on the communication path thus set up.
With ATM switches, a point-to-multipoint communication path connection function is provided for implementing a video-on-demand (VOD) service (a service that delivers information from the source to a subscriber on demand). In VOD, communication service is provided to a subscriber upon the subscriber newly registering for the VOD service.
In conventional ATM switches, for a point-to-multipoint communication path, only a test cell continuity check function to check continuity within the ATM switch providing the point-to-multipoint connection, and an OAM cell loopback test function to perform a loopback test from a root-side subscriber to a root-side path connection point of the ATM switch providing the point-to-multipoint connection, and from a leaf-side ATM switch to a leaf-side path connection point of the ATM switch providing the point-to-multipoint connection, have been provided separately, and it has not been possible to perform a cell continuity test from a root-side path connection point of the ATM switch providing the point-to-multipoint connection to a subscriber-side path connection point of the ATM switch accommodating a leaf-side subscriber. Accordingly, when locating a fault that has occurred on a point-to-multipoint communication path, or when performing a cell flow check on a communication path added on the leaf side, it has been necessary to perform two patterns of test, one within the ATM switch providing the point-to-multipoint connection (between the root-side path connection point and each of the leaf-side path connection points) and the other between each of the leaf-side path connection points of the ATM switch providing the point-to-multipoint connection and the leaf-side ATM switch. This has complicated maintenance work.
Furthermore, in conventional ATM switches, for a communication path of a dual configuration, an OAM cell loopback test function has been provided only for the communication path currently used for communication (working communication path), and an OAM cell loopback test function for a spare communication path (protection communication path) has not been provided. As a result, when switching is made in the dual communication path to the protection path because of a failure of the working path, continuity may not be achieved since the quality of the protection path has not been checked periodically.